Here is what the CPS (Canadian PDR) says about metronidazole in its
"Drug Exposure during Lactation" section:

"Metronidazole has been cited as a drug contraindicated during
lactation.  This advice is based on reports that the drug is mutagenic
in bacteria and carcinogenic in rodents when taken throughout their
lives.  Specific untoward effects in a nursing infant as a result of
metronidazole ingestion have not been reported.  Without more direct
evidence of the harmful effects of short term use in humans it seems
overly conservative to withhold the drug or discontinue
breastfeeding..."

Furthermore, we use metronidazole in babies.

Furthermore, the risks of *not* breastfeeding way outweigh the risks
of a little metronidazole.

Jack Newman, MD, FRCPC